A magnetic recording medium (particularly for an audiocassette tape which has recently been used for recording music) must have a high output level at high frequency (i.e., good frequency characteristics) as well as good reproducing ability with respect to recorded sound. Furthermore, since car stereos and radios with audiocassette recorders are becoming very common, the tapes which are repeatedly used within them must have good running properties and durability.
In a videocassette tape recording system, the wave length to be recorded is made shorter, and the track width is made narrower to achieve high density recording. Therefore, it is necessary for a tape to have a high output level and reproducing ability of an original image with a high S/N ratio. It is also necessary for a recording tape for video tape recorder (VTR), with the increase of commercial use of portable VTR, that the thickness of video tapes be 20 .mu.m or thinner. Furthermore, the running properties and durability of the tapes have to be better than conventional video tapes. That is, audio tapes and video tapes having improved surface property, orientation, running properties and durability are required.
Various binder compositions have been proposed to improve the properties. However, no binders have been found which completely satisfy the above requirements.
Presently, binders are comprised of a mixture of a rubber type resin and a copolymer of vinyl chloride/vinyl acetate/other vinyl compound. Such binders give satisfactory results with respect to dispersibility of the ferromagnetic powder, running properties and durability of the magnetic recording layer. Generally, a copolymer of vinyl chloride/vinyl acetate/vinyl alcohol and a copolymer of vinyl chloride/vinyl acetate/maleic anhydride are used as a copolymer of vinyl chloride/vinyl acetate/other vinyl compound. Particularly, a copolymer of vinyl chloride/vinyl acetate/vinyl alcohol is often used because such a binder makes it possible to obtain excellent dispersibility of the ferromagnetic powder.
A conventionally used copolymer of vinyl chloride/vinyl acetate/vinyl alcohol is composed of 91 to 95% of vinyl chloride, 3 to 6% of vinyl acetate and 2 to 3% of vinyl alcohol. It is commercially available under the trade-name of VAGH manufactured by Union Carbide Co., Ltd., SLEC A manufactured by Sekisui Kagaku Kogyo Co., Ltd., and Denka Vinyl 1000G manufactured by Denki Kagaku Kogyo Co., Ltd.
However, the use of conventional copolymers of vinyl chloride/vinyl acetate/vinyl alcohol results in poor plasticity of the magnetic recording layer. Accordingly, it is necessary that a large amount of plasticizer or soft resin be incorporated in the copolymer. However, if a large amount of plasticizer is used, blooming readily occurs and the durability of a magnetic recording layer degrades. Further, if a large amount of soft resin is used in combination with the copolymer, the dispersibility of a ferromagnetic powder degrades and the running properties also degrade at high temperature and high humidity.
Accordingly, when a copolymer of vinyl chloride/vinyl acetate/vinyl alcohol is used, it is not possible to obtain a magnetic recording medium having satisfactory electromagnetic properties, running properties and durability.